Field of Invention
The present invention relates to a method of forming a composite substrate. A semiconductor light emitting device such as a III-nitride light emitting device may be grown on the composite substrate.
Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
FIG. 1 illustrates a composite substrate for growing a III-nitride structure, described in more detail in US 2007/0072324, which is incorporated herein by reference. Substrate 10 includes a host substrate 12, a seed layer 16, and a bonding layer 14 that bonds host 12 to seed 16. Host layer 12 may be, for example, sapphire or Si and bonding layer 14 may be, for example, SiOx or SiNx. Seed layer 16 may be, for example, an InGaN layer grown strained on a conventional substrate such as sapphire, then bonded to a host 12 and released during the process from the growth substrate such that the InGaN seed layer at least partially relaxes. Providing the seed layer as stripes or a grid over bonding layer 14, rather than as a single interrupted layer, may result in further strain relief. For example, seed layer 16 may be formed as a single uninterrupted layer, then removed in places, for example by forming trenches, to provide strain relief.
III-nitride seed layer materials may require additional bonding steps in order to form a composite substrate with a III-nitride seed layer in a desired orientation. Wurtzite III-nitride layers grown on c-plane sapphire or c-plane SiC growth substrates are typically grown with a c-plane orientation; e.g. the so-called “c-planes” of the III-nitride layers and the substrates are parallel to each other. Such c-plane wurtzite III-nitride structures have a gallium face and a nitrogen face. III-nitrides are well known to grow with the highest crystalline quality (as measured by luminescence efficiency) when the top surface of the grown layer is the group-III face, often referred to for economy of language as the “gallium face” or “Ga-face,” even though it need not include gallium. The bottom surface (the surface adjacent to the growth substrate) is the nitrogen face or “N-face”. For example, Masui, et al. describe in “Luminescence Characteristics of N-Polar GaN and InGaN Films Grown by Metal Organic Chemical Vapor Deposition”, Japanese Journal of Applied Physics 48, 071003 (2009) that the luminescence efficiency of N-face InGaN films grown by metal-organic chemical vapor deposition (MOCVD) is less than the luminescence efficiency of Ga-face InGaN films grown by MOCVD. Simply growing seed layer material conventionally on sapphire or SiC then connecting the seed layer material to a host and removing the growth substrate would result in a composite substrate with a III-nitride seed layer with the nitrogen face exposed. III-nitrides preferentially grow on the gallium face, i.e. with the gallium face as the top surface, thus growth on the nitrogen face may undesirably introduce defects into the crystal, or result in poor quality material as the crystal orientation switches from an orientation with the nitrogen face as the top surface to an orientation with the gallium face as the top surface.
To form a composite substrate with a III-nitride seed layer with the gallium face as the top surface, seed layer material may be grown conventionally on a growth substrate, then bonded to any suitable intermediate substrate, then separated from the growth substrate, such that the seed layer material is bonded to the intermediate substrate through the gallium face, leaving the nitrogen face exposed by removal of the growth substrate. The nitrogen face of the seed layer material is then bonded to a host substrate 12, the host substrate of the composite substrate. After bonding to the host substrate, the intermediate substrate is removed by a technique appropriate to the growth substrate. In the final composite substrate, the nitrogen face of the seed layer material 16 is bonded to host substrate 12 through optional bonding layer 14, such that the gallium face of III-nitride seed layer 16 is exposed for growth of epitaxial device layers.